In micro-electromechanical systems (MEMS) (e.g., atomic resolution storage devices, vacuum microelectronic devices, miniature x-ray sources, and other such), it is desirable to hermetically encapsulate devices within a near vacuum. Micro-optical electromechanical systems (MOEMS) require a vacuum for reliable operation. Typically, the operational life of a device is reduced when the vacuum is not maintained. Thus, it is desirable to maintain the vacuum within the device.
Semiconductor, other electronic and mechanic devices, such as MEMS, MOEMS and other similar devices, are often hermetically encapsulated as a package in such a way as to provide a near vacuum within the device. Although these packages are hermetically sealed, outgassing (release of gasses from a solid as a result of heating or reduced pressure) from a number of sources within the package releases moisture and gasses that decrease the operational life of the encapsulated devices by reducing the internal vacuum. Encapsulated packages also allow gasses to diffuse through their encapsulation materials and/or may have micro-leaks that, over time, allow gases to enter the encapsulation.
One solution to this problem is to include a getter material that absorbs and traps any outgased substances. For example, MOEMS devices often include getters that selectively attract undesirable substances within the hermetic encapsulation, thereby prolonging the operational life of the device.
Evaporated getters and activated getters are typically based on barium (Ba), titanium (Ti), zirconium (Zr), vanadium (V), iron (Fe) and aluminum (Al) alloys that react with gas molecules to trap them. Typically, such getters require high outgassing and activating temperatures. More specifically, a getter may require heating (typically=400° C.) using a certain heating method for a certain period of time under a near vacuum to achieve optimum activation. Evaporated deters are typically used due to their simplicity. They are sputtered after sealing and generally require a lot of mirror surfaces for the gas absorption. In addition, they may leak out, diffuse into the device or in other ways fail to perform as expected. For small package environments, especially micro-package environments, evaporated getters are usually inappropriate. Activated getters are typically must valuable when used for small vacuum shells.
Typically, the vacuum must be maintained during the cooling off period of the getter, prior to sealing the encapsulation. Additionally, some getter types have a certain operating temperature, and may thus require additional heating during operation in order to be affective. This temperature activation, particularly during operation, causes additional stress to the encapsulated device, and is inappropriate for small volumes desired to be at a near vacuum. Further, once activated, getter materials have a limited life, absorbing only limited amounts of gasses chemically active gasses such as O2, H2O, CO, CO2, and etc.
In one example, a micro-resonator device requires a controlled, low-pressure or vacuum environment for high Q factor operation (Q factor is a measure of the “quality” of a resonant system and is defined as the resonant frequency divided by the bandwidth). A typical mass for a very high frequency (VHF) micro-resonator is approximately 10−13 kilograms, and thus small amounts of mass-loading (e.g., from gas molecules) cause significant resonance frequency shifts and induce phase noise. It is thus desirable to maintain and measure gas pressure within the micro-resonator's environment to ensure correct operation. There is currently no method of measuring pressure in volumes less than 0.5 cm3.
Ion pumps are typically used to create a near vacuum and operate by ionizing gas within a magnetically confined, cold cathode discharge. Electrons, produced by the cold cathode discharge, are entrapped within a magnetic field and collide with gas molecules to form ions. Typically, the cathode of an ion pump is comprised of titanium. These ions are accelerated towards a titanium cathode, where they sputter titanium. The sputtered titanium chemically reacts with, and traps, active gasses, and the sputtered titanium buries other noble gasses on impact with the pump walls.
For example, an ion pump may be used to create a vacuum during getter activation prior to device encapsulation, where the entire encapsulation process is being performed within the vacuum.
To increase the longevity and operational life expectancy of a vacuum-dependent device, it is desirable to provide continued evacuation after original encapsulation. In addition, a measurement of internal pressure may be used to predict operational performance.
As stated above, although the encapsulated environment is initially created with a vacuum, the vacuum typically degrades with time. It is generally impractical to re-evacuate the package environment by performing a re-encapsulation or by connecting the package to an external vacuum pump.
Hence, there is a need for a vacuum micropump and gauge that overcomes one or more of the drawbacks identified above.